1. Field of the Invention
The invention relates, in general, to a speed control system for a rotating shaft prime mover, and in particular, relates to a speed sensing system and method for detecting a failure in the speed sensing circuit.
2. Background Discussion
Prime movers operate under the control of large inline fluid control valves which regulate the flow of motive fluid into the prime mover. These fluid control valves may include a stop valve and a control valve positioned in series in a steam header upstream from the turbine. The control valve may be set to any intermediate postion between full open and full closed, whereas the stop valve is usually either full open or full closed.
In this type of system, the control of a turbomachine is well suited for a feedback control system which measures the rotating shaft speed. In a turbomachine, electronic speed control is provided for both normal operating conditions and for overspeed protection. Under normal operating conditions turbine control valves are positioned in accordance with a comparison between a desired speed and actual speed. A range of control is maintained from 1%-100% of rated speed. In response to a valve position change command, the turbine valve is adjusted to a more open or more closed position.
An overspeed control relates to overspeed conditions which pertains to speeds in excess of the rated speed. When the rated speed is exceeded, the overspeed control causes the turbine to trip (shutdown) based on the rapid shutting down of motive fluid to the turbine. Overspeed protection is usually provided in the form of a solenoid valve between the source of the motive fluid and the turbine. The solenoid valve is normally full open during normal operating conditions or fully close in response to a signal from the overspeed control circuit.
A loss of signal control relates to a loss of signal in the speed control for normal operating conditions and overspeed conditions. A failure in any of the speed pickup sensors or the associated speed sensing circuitry could cause a false overspeed condition. A speed monitoring system for monitoring the sensors and sensing circuitry must notify the operator of a sensor signal failure, detect the nature of the failure, and if possible allow the turbine to continue operating. It is this type of speed control to which this invention is directed.
Some turbines are used in support of operations such as boiler feed pump turbines, industrial drive turbines, and plant process turbines. The underlying common concern regarding these types of support turbines is that a shutdown of the turbine due to an overspeed condition or turbine control failure may cause a train of shutdown events which can be catastrophic. Therefore, it is desirable to trip the turbine machine when a true overspeed condition arises, and not to trip the turbine machine unless an overspeed condition actually exists.
Numerous examples relating to speed protective circuits of a prime mover are available in the prior art including U.S. Pat. Nos. 4,464,577 and 4,217,617. Briefly, a plurality of redundant speed pickup sensors provide a primary speed signal. The speed signals are input into one of three identical speed translators which provide a loss of signal output. The loss of signal output is generated whenever the actual speed signal falls below some low threshold value. A problem with this type of circuitry is that the loss of signal is only detected upon the failure of a speed pickup sensor. If the loss of signal occurs further within the circuitry, there is no indication of a loss of signal. If a loss of signal does occur further within the circuitry, a false overspeed condition may arise with disasterous results causing a shutdown to the turbine which can be very costly to the turbine owner.
At startup, the speed pickup sensors measure a zero speed which may trigger the overspeed protection circuitry. When a turbine is started, time delay circuitry is used to defeat the overspeed protection circuitry. When the time delay circuit times out, it activates the overspeed protection circuitry. A problem with the time delay circuit is that the turbine may have different loads at startup. This causes the turbine to reach normal operating speeds at different times which can not be accurately predicted. If the time delay is too short, the overspeed protection may trip too early causing an inability to start the turbine machine. If the time delay is too long, the turbine machine may reach operating speed with no overspeed protection while the time delay circuitry is active.
Therefore, it is an object of the present invention by providing a speed sensing system that eliminates the previous shortcomings of the startup time delay circuit.
It is another object of the present invention to provide a more reliable loss of speed sensing circuitry than has been heretofore available to turbine owners.
It is yet another object to provide a means and method for determining the loss of speed signal in one speed pickup sensor and automatically switching to another speed pickup sensor while continuing to provide loss of speed protection.
It is a further object to provide redundant speed sensing circuitry in order to avoid unnecessary turbine shutdown.
It is another object of the present invention to provide loss of speed protection in the entire speed pickup sensing system.